Combustion engine with double-ended pistons and transfer passages

ABSTRACT

An engine block is provided having a crankshaft journaled therethrough including at least one pair of longitudinally spaced eccentric crankpin portions. A double-ended piston has its midportion journaled on each pin portion and the pistons are slidable in a pair of cylinder bodies supported in the block for reciprocal sliding therein transversely of the crankshaft and the cylinder bodies. A pair of intake ports are formed in the block and the opposite ends of a first cylinder includes end walls having inlet openings formed therethrough registrable with the intake ports, and also outlet transfer openings registrable with the inlet ends of transfer passages in the block having outlet ends with which the inlet transfer openings in end walls carried by the second cylinder are registrable. Also, the second cylinder end walls further include outlet openings registrable with exhaust ports formed in the block and the transfer passages include ignition structure. The various openings formed in the cylinder end walls are registrable with the corresponding ports and opposite ends of the transfer passage in timed sequence to support (1) the alternate intake of a combustible air and fuel mixture in the opposite ends of the first cylinder, (2) the alternate discharge of the combustible air and fuel mixture from the first cylinder into the transfer passages on successive strokes of the first cylinder piston, (3) the burning of the combustible mixture in the transfer passages (4) the admission of the burning mixture into the opposite ends of the second cylinder (5) and the timed registry of the outlet openings of the second cylinder body with the exhaust ports to exhaust the by-products of combustion from the second cylinder body in correct timed sequence. Also, structure is provided whereby the available volume of the transfer passage in which ignition of the combustible air and fuel mixture takes place may be varied so that the effective compression ratio at the time of ignition may be varied.

United States Patent 191 White 1 Apr. 22, 1975 1 1 COMBUSTION ENGINE WITH DOUBLE-ENDED PISTONS AND TRANSFER PASSAGES [76] lnventor: Norman C. White, 2501 Bella Vista Ave., Martinez, Calif. 94553 {22] Filed: Nov. 15, 1973 [21] Appl. No: 416,264

[52] U.S. Cl. 123/61 R; 123/70 R [51] Int. Cl. F02b 75/40 [58] Field of Search 123/45 R, 65 VD, 61 V, 123/70 R, 70 V, 61 R [56] References Cited UNITED STATES PATENTS 652,724 6/1900 Pickles et a1. 123/61 R 750,336 l/l904 Bentz 123/197 R 1,046,863 12/1912 Saunders et al.... 123/61 R 1,645,795 10/1927 Burtnett 123/191 S 2,130,037 9/1938 Skzirlund..." 417/466 X 2,454,138 11/1948 Delzer 123/61 R X 2,504,945 4/1950 Austin et a1. 417/466 X 3,241,745 3/1966 Williams 417/462 Primary ExaminerCharles J. Myhre Assistant Examiner-William C. Anderson Attorney, Agent, or FirmClarence A. O'Brien; Harvey B. Jacobson [5 7] ABSTRACT An engine block is provided having a crankshaft jour naled therethrough including at least one pair of longitudinally spaced eccentric crankpin portions. A double-ended piston has its mid-portion journaled on each pin portion and the pistons are slidable in a pair of cylinder bodies supported in the block for reciprocal sliding therein transversely of the crankshaft and the cylinder bodies. A pair of intake ports are formed in the block and the opposite ends of a first cylinder includes end walls having inlet openings formed therethrough registrable with the intake ports, and also outlet trans fer openings registrable with the inlet ends of transfer passages in the block having outlet ends with which the inlet transfer openings in end walls carried by the second cylinder are registrable. Also, the second cylinder end walls further include outlet openings registrable with exhaust ports formed in the block and the transfer passages include ignition structure. The various openings formed in the cylinder end walls are registrable with the corresponding ports and opposite ends of the transfer passage in timed sequence to support (1) the alternate intake of a combustible air and fuel mixture in the opposite ends of the first cylinder, (2) the alternate discharge of the combustible air and fuel mixture from the first cylinder into the transfer passages on successive strokes of the first cylinder piston, (3) the burning of the combustible mixture in the transfer passages (4) the admission of the burning mixture into the opposite ends of the second cylinder (5) and the timed registry of the outlet openings of the second cylinder body with the exhaust ports to exhaust the by-products of combustion from the second cylinder body in correct timed sequence. Also, structure is provided whereby the available volume of the transfer passage in which ignition of the combustible air and fuel mixture takes place may be varied so that the effective compression ratio at the time of ignition may be varied.

8 Claims, 11 Drawing Figures COMBUSTION ENGINE WITH DOUBLE-ENDED PISTONS AND TRANSFER PASSAGES BACKGROUND OF THE INVENTION The combustion engine ofthe instant invention. in its simplest form. utilizes a pair of double-ended pistons reciprocal in sliding cylinders and the opposite ends of one of the cylinders functions as an air compressor for compressing a combustible mixture of air and fuel and the opposite ends of the other cylinder comprise expansion chambers in which the combustible mixture may expand after ignition. The engine includes transfer passages which communicate corresponding ends of the cylinders and into which the compressed air and fuel mixture from the first cylinder is pumped. each of the transfer passages also including ignition means. Any suitable structure may be provided for timing operation of the ignition means and the sliding cylinders include end walls having inlet and outlet openings therein successively registrable with the various intake ports, exhaust ports and transfer passage ends in proper timed sequence. By this type of construction one of the cylinders is subject to relatively cool intake air and fuel mixtures and the combined combustion chamber and transfer passages are subject only to compressed and thus heated air and fuel mixtures and the heat of combustion of the air and fuel mixture and the other cylinder is subject only to the heat of combustion. Inasmuch as the combined combustion and transfer passages are not subject to cooling by the intake of fresh air and fuel mixtures before compression and the expansion chambers defined in the opposite ends ofthe second cylinder are subject only to the heat of combustion. the engine is capable of substantially complete combustion with low emission of air pollutants. Further. inasmuch as even the simplest form of invention utilizing two sliding cylinders having double-ended pistons reciprocal therein experiences two power strokes per revolution. the expected power output and smoothness of opera tion of the engine is equal to a two cylinder two cycle engine or a four cylinder four cycle engine and thus the power-to-rate ratio is relatively high.

In addition. in an attempt to minimize maintenance problems the engine has been designed with only five moving major components, which components comprise the two double-ended pistons. the two sliding cylindcrs and the crankshaft.

The main object of this invention is to provide a combustion engine which will operate at high efficiency.

Yet another object of this invention is to provide a combustion engine which is inherently of the super charged type and which will therefore enjoy a high power-to-rate ratio.

Yet another object of this invention, in accordance with the immediately preceding object. is to provide a combustion engine of the reciprocating piston type including only two pistons and yet which will develop two power strokes per revolution thereby further enhancing the power-to-weight ratio of the engine.

Another important object of this invention is to provide a reciprocating piston internal combustion engine designed in a manner such that the exhaust therefrom will be relatively free of pollutants.

A further object of this invention is to provide a reciprocating piston combustion engine designed in a manner whereby the basic engine will include a minimum of major moving components and enlargement of the engine to double its basic size involves the addition of only four further major moving components.

A final object of this invention to be specifically enumerated herein is to provide a combustion engine in accordance with the preceding objects which will con form to conventional forms of manufacture. be of sim ple construction and easy to operate so as to provide a device that will be economically feasible. long lasting and relatively trouble-free in operation.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed. reference being had to the accompanying drawings forming a part hereof. wherein like numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a perspective view of the combustion engine of the instant invention;

FIG. 2 is an enlarged top plan view of the combustion engine;

FIG. 3 is an enlarged transverse vertical sectional view taken substantially upon the plane indicated by the section line 33 of FIG. 2;

FIG. 4 is a transverse vertical sectional view taken substantially upon the plane indicated by the section line 44 of FIG. 2:

FIG. 5 is an enlarged longitudinal vertical sectional view taken substantially upon the plane indicated by the section line S5 of FIG. 2:

FIG. 6 is an exploded perspective view of one of the double-ended pistons of the combustion engine:

FIG. 7 is a perspective view of one of the sliding cyl inder assemblies of the combustion engine;

FIG. 8 is an enlarged transverse vertical sectional view taken substantially upon the plane indicated by the section line 88 of FIG. 2; and

FIGS. 9. l0 and II are three transverse vertical sectional views taken substantially upon a plane passing through the center of the air and fuel mixture compression cylinder of the engine illustrating the sliding cylinder and associated piston and crankshaft in successive positions of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more specifically t0 the drawings. the numeral I0 generally designates the combustion engine ofthe instant invention. The engine I0 includes a cylinder block I2 having opposite end walls I4 and 16 interconnected by means of opposite side walls 18 and 20. The top and bottom of the cylinder block 12 are open and removably closed by means of top and bottom cylinder heads 22 and 24 removably secured over the top and bottom of the cylinder block 12 by means of suitable fasteners 26.

The opposing inner surfaces of the cylinder heads 22 and 24 define planar and parallel bearing surfaces and a pair of cylinder assemblies referred to in general by the reference numerals 28 are slidingly disposed within the cylinder block I2 for transverse movement between the side walls 18 and 20. Each cylinder assembly includes a cylinder member 30 having opposite end rectangular end flanges 32 formed integrally therewith and through which the opposite ends of the cylinder member 30 open. In addition. each of the cylinder members includes centrally located opposite side diametrically registered peripheral slots or openings 34 and the juncture between each cylinder member end and the corresponding end flange 32 defines a counterbore 36.

As can best be seen from FIGS. 3. 4 and S of the drawings each cylinder member end includes a circular end wall 38 snugly recessed within the corresponding counterbore 36 and relieved about its outer periphery as at 40 to seatingly receive a circular sealing ring 42. Each end wall 38 includes an inlet opening 44 formed therethrough and an outlet opening 46 formed therethrough. From FIG. 2 3 and 4 of the drawings it may be seen that the inlet and outlet openings 44 and 46 formed in the end walls 38 of the left hand cylinder assembly 28 are elongated transversely of the engine 10. that the inlet opening 44 in the end wall 38 ofthe right hand cylinder assembly 28 is elongated transversely of the engine 10. and that the outlet openings 46 formed in the end walls 38 of the right hand cylinder assembly 28 are cylindrical.

The end walls l4 and 16 have vertically elongated openings 54 and 56 formed therein and the end walls l4 and [6 include cover plates 58 and 60 secured thereto by means of fasteners 62. The end walls 14 and 16 additionally include integral lower half bearing bosses 64 and 66 and the corresponding cover plates 58 and 60 are also secured to the bearing bosses 64 and 66 by means of fasteners 68. Each bearing boss 64 and the corresponding cover plate 58 defines a journal structure in which a bearing assembly 70 is secured and the opposite end portions of a crankshaft 72 are journaled in the bearing assemblies 70. The crankshaft 72 includes a pair of longitudianlly spaced eccentric crankpins 74 and 76 and the pins 74 and 76 are spaced apart by means of a center portion 78 of the crankshaft 72 aligned with the opposite end portions of the crank' shaft journaled through the bearing assemblies 70 and the crankshaft 72 is removable and insertable through the opening 54 or the opening 56.

A pair of double-ended piston assemblies referred to in general by the reference numerals 80 and 82 are provided and each assembly 80 and 82 includes a pair of piston end sections 84 equipped with piston rings 86 and defining half bearing bosses 88 having half cylindrical bearing inserts 90 seated therein The piston sections 84 of each piston assembly are secured together by means of longitudinally extending fasteners 92 in a manner which is believed to be obvious from FIG. 6.

The piston assemblies 80 and 82 are secured about and thus journaled on the crankpin 74 and 76 with the opposite end portions of the crankshaft 72 and the center portion 80 swingable through opposite side windows 94 defined in each piston assembly 80 and 82 and slidably received through the adjacent slots 34.

The cylinder assemblies 28 are snugly received be tween the cylinder heads 22 and 24 and the remote side edges of the end flanges 32 of the cylinder assemblies 28 are slidably engaged with the opposing inner surfaces ofthe end walls 14 and 16 while the adjacent side edges ofthe end flanges of adjacent cylinder assemblies 28 are slidably engaged with each other.

The piston sections 84 of the piston assembly 80 include oval projections 98 which are snugly receivable in the corresponding inlet and outlet openings 44 and 46 and the piston sections 84 of the piston assembly 82 include oval projections I00 which are snugly seatable in the corresponding openings 44 and cylindrical projections 102 which are snugly receivable in the corre sponding openings 46.

Each of the cylinder heads includes an integral flanged intake passage portion 104 to which an air and fuel mixing device such as a carburetor may be attached at the flanged inlet end of the portion 104. Each portion 104 terminates inwardly at the outlet end 106 thereof with which the corresponding inlet opening 44 is registrable. Further. each cylinder head includes portion 108 defining a combined combustion and transfer passage 109 including an inlet end 110 having a valve seat 112 secured therein with which a spring-biased poppet valve H4 is operatively associated and with which the outlet opening 46 of the corresponding end wall 38 of the piston assembly 80 is registrable. Each passage [09 further includes an outlet end H6 with which the inlet opening 44 of the corresponding end wall 38 of the piston assembly 82 is registrable. Finally, each cylinder head also defines an exhaust gas outlet passage defining portion 118 which is flanged on its outlet end for attachment to a suitable exhaust system and which includes an inlet end or opening 120 with which the outlet opening 44 of the corresponding end wall 38 of the piston assembly 82 is registrable.

The cylinder assembly 28 at the right hand side of FIG. 5 differs from the cylinder assembly on the left hand side of FIG. 5 in that the left hand side thereof includes integral upper and lower exhaust passages 124 which open through the adjacent sides of the cylinder member as at [26 at their inlet ends and as at 128 at their outlet ends into initial exhaust ports 130 formed in the cylinder heads 22 and 24. The cylinder heads 22 and 24 further include spark plugs 132 communicated with the mid-portions of the passages I09 and each portion 108 includes a portion [34 defining a cylindrical branch passage 136 opening into the mid-portion of the corresponding passage 109. The a cylindrical branch passage 136 have pistons 138 reciprocal therein carried by tubular piston rods 140 and the rods 140 are provided with ports 142 adjacent the pistons 138 and ports [44 in portions thereof slidable through bores 146 formed in valve bodies 148 having reciprocal control rods [50 anchored thereto.

Each valve body 148 includes an inlet port 152 which opens into the corresponding bore 146 and an outlet port 154 which opens into the bore 146 a spaced distance from the inlet port 152. Fluid supply and return lines 156 and [58 are connected to the ports 152 and 154 at one pair of ends and are communicated with a suitable source (not shown) of fluid under pressure and return reservoir therefor at their other ends. The spac' ing between the ends of the ports 152 and 154 which open into the bore 146 is greater than the diameter of the port 144 whereby the latter may be spaced between the ports 152 and 154 as illustrated in FIG. 8 of the drawings. If the control rod l50 is shifted toward the left as viewed in FIG. 8 of the drawings the port 152 is registered with the port 144 and fluid under pressure from the supply line 156 is ducted through the port l52 and the hollow piston rod 140 and into the cylindrical passage 136 through the port 142 behind the pistons 138 in order to cause the piston 138 to shift toward the left as viewed in H0. 8 of the drawings. This will in effect reduce the total volume of the combined combustion and transfer defining passage 108. On the other hand. if the control rod is shifted to the right as viewed in FIG. 8 of the drawings the port 154 will be registered with the port 144 and fluid behind the piston [38 will be vented to the return line 158 to thus enable the piston l38 to move to the right in order to increase the effective volume of the combined combustion and transfer passage defining portion 108. Of course. as fluid is vented from behind the piston [38 and the piston 138 shifts to the right. the port 144 is moved out of registry with the port 154 and disposed between the ports 152 and 154 to terminate further movement of the piston [38 until the control rod 150 is again longitudinally shifted.

With attention now invited more specifically to FIG. 9 through 11 of the drawings it may be seen that the piston assembly 80 in PK]. 9 is on the downward stroke with the upper inlet opening 44 registered with the inlet opening 106 and the lower outlet opening 46 registered with the inlet end 110 of the lower combined combustion and transfer passage defining portion 108. Thus. a combustible mixture of air and fuel is being drawn into the upper portion of the corresponding cylinder assembly 28 and a combustible mixture of air and fuel is being pumped from the lower end of the corresponding cylinder assembly 28 into the combined combustion and transfer passage defining portion 108. In FIG. [0. the piston assembly 80 has reached its lowermost position and the upper inlet opening 44 has moved out of registry with the outlet end of the upper intake port de fining portion 104 and the lower outlet opening 46 is moved out of registry with the inlet end 110 of the lower combined combustion and transfer passage de fining portion [08. At this point. the projections 98 on the piston assembly 80 are fully seated in the lower openings 44 and 46 and in FIG. 11 of the drawings the lower inlet opening is registered with the outlet end of the lower intake port portion 104 and the upper outlet opening 46 is registered with the inlet end 110 of the upper combined combustion and transfer passage defining portion 108. Accordingly. when the piston assembly 80 is positioned as illustrated in FIG. 11 of the drawings, a fresh mixture of air and fuel is being drawn into the lower end of the corresponding cylinder assembly 28 and the combustible mixture previously drawn into the upper end of the cylinder assembly 28 is being forced therefrom into the upper combined combustion and transfer passage defining portion 108. Of course, in this manner successive quantities of combustible air and fuel mixture are pumped into the upper and lower portions 108 for ignition therein by the spark plugs 32. Of course, the poppet valves 114 prevent the backflow of compressed air and fuel mixture from the portions 108 into the upper and lower ends of the cylinder as sembly 28 in which the piston assembly 80 is reciprocal.

Also, it will be understood that the operation of the piston assembly 82 and corresponding cylinder assembly 28 is substantially identical. except that as the piston assembly 82 begins its downward stroke the corresponding upper inlet opening 44 registers with the outlet end 116 of the upper passage defining portion [08 to admit the burning air and fuel mixture into the upper end of the cylinder assembly in order to force the piston assembly 82 downward. During downward stroke of the piston assembly 82 the lower outlet opening 44 is registered with the inlet end of the lower exhaust passage defining portion 8 in order that the exhaust gases in the lower portion of the cylinder assembly may be exhausted therefrom through the lower exhaust gas outlet port defining portion 118.

In order to initially relieve the upper and lower ends of the cylinder assembly 28 in which the piston assembly 82 is reciprocal at the opposite ends of the upward and downward strokes of the piston 82, the exhaust passages [24 are provided and are uncovered by the piston assembly 82 at the ends of its strokes. Thus, initial exhausting ofexhaust gases from the opposite ends of the cylinder assembly 28 in which the piston assembly 82 is reciprocal is carried out via the exhaust ports I24 which open into the ports [30 and thereafter into the exhaust gas outlet port defining portions H8.

The engine 10 is able to operate on a variety of fuels due to the ability to vary the effective volume of the passages 108 which define precombustion chambers. Further the power-to-weight ratio of the engine 10 is considerably higher than is usual for a reciprocating piston combustion engine due to the fact that two power strokes are realized from only two cylinders for every revolution of the crankshaft 72. In addition, the combustible mixture pumped into the passage defining portions 108 are preheated by compression and thus the passage defining portions 108 are not subject to cooling by the intake of fresh outside air. Further. the cylinder assembly 28 in which the piston 82 is reciprocal is subject only to the temperatures of combustion and therefore more complete combustion is realized and the exhaust gases of the engine 10 are reasonably free of pollutants.

The foregoing is considered as illustrative only of the principles of the invention. Further. since numerous modifications and changes will readily occur to those skilled in the art. it is not desired to limit the invention to the exact construction and operation shown and described. and accordingly all suitable modifications and equivalents may be resorted to. falling within the scope of the invention.

What is claimed as new is as follows:

1. A combustion engine including a block structure from which a crankshaft including a pair of axially spaced crankpin portions is journaled. a pair of cylinder assemblies guidingly supported from said block structure with said assemblies extending normal to the axis of rotation of said crankshaft and said assemblies mounted for guided lateral shifting transversely of said crankshaft. said cylinder assemblies including central opposite side peripherally extending slots through which axial portions of said crankshaft pass, a pair of double-ended pistons reciprocal in said cylinders and journaled on said crankpin portions. said cylinder assemblies each including opposite end walls through each of which inlet and outlet openings are formed. said block structure including opposing portions between and relative to which the opposite end walls of said cylinder assemblies are slidable, each of said opposing portions including l an intake port with which the corresponding inlet openings of one of said assemblies are registrable, (2) an exhaust port with which the corresponding outlet openings of the other of said assemblies are registrable, (3) and a combined compression. combustion and transfer passage including inlet and outlet ends with which the outlet and inlet open ings of the corresponding end walls of said one and other cylinder assemblies, respectively, are registrable. each of said opposing portions of said block including volume varying means operative to adjustably vary the effective volume of the corresponding transfer passage.

2. The combination of claim 1 wherein said opposing portions comprise removable cylinder head portions of said block structure.

3. The combination of claim 1 wherein said other cyl inder assembly includes integral opposite end exhaust outlet passages including inlet ends opening through a side wall portion of said other assembly at their inlet ends. said opposing portions including connecting eshaust ports having inlet ends and outlet ends opening into the first-mentioned exhaust ports. the outlet ends of said exhaust outlet passages being registrable with the inlet ends of said connecting exhaust ports. the inlet ends of said exhaust outlet passages being positioned to be uncovered by the opposite ends of the double-ended piston in said other cylinder assembly. said opposite ends closely approaching their inwardmost limits of movement away from the adjacent opposing portions of said block structure.

4. The combination of claim I wherein the crankpin portion upon which the piston in said one cylinder as sembly closely trails the other crankpin portion relative to the direction ofintended rotation of said crankshaft.

5. The combination of claim I wherein the inlet ends of said transfer passages include check valve means for preventing reverse flow of compressed gases from said transfer passages into said one cylinder assembly.

6. The combination of claim I wherein each of said transfer passages includes a branch passage intermediate its opposite ends. said branch passages each having a piston slidably disposed therein, said volume varying means comprises shifting means operative to shift said pistons in said branch passages toward and away from the ends thereof opening into said transfer passages.

7. The combination of claim 6 wherein said shifting means includes fluid supply and vent means operative to selectively admit fluid under pressure into and vent fluid pressure from the portion of each of said branch passages behind the pistons therein.

8. The combination of claim 7 wherein said fluid supply and vent means includes a shiftable actuator and means operative to admit fluid into and vent fluid from the corresponding branch passages such to establish a predetermined position of said pistons in said branch passages in response to said actuator being shifted to corresponding predetermined position 5. 

1. A combustion engine including a block structure from which a crankshaft including a pair of axially spaced crankpin portions is journaled, a pair of cylinder assemblies guidingly supported from said block structure with said assemblies extending normal to the axis of rotation of said crankshaft and said assemblies mounted for guided lateral shifting transversely of said crankshaft, said cylinder assemblies including central opposite side peripherally extending slots through which axial portions of said crankshaft pass, a pair of double-ended pistons reciprocal in said cylinders and journaled on said crankpin portions, said cylinder assemblies each including opposite end walls through each of which inlet and outlet openings are formed, said block structure including opposing portions between and relative to which the opposite end walls of said cylinder assemblies are slidable, each of said opposing portions including (1) an intake port with which the corresponding inlet openings of one of said assemblies are registrable, (2) an exhaust port with which the corresponding outlet openings of the other of said assemblies are registrable, (3) and a combined compression, combustion and transfer passage including inlet and outlet ends with which the outlet and inlet openings of the corresponding end walls of said one and other cylinder assemblies, respectively, are registrable, each of said opposing portions of said block including volume varying means operative to adjustably vary the effective volume of the corresponding transfer passage.
 1. A combustion engine including a block structure from which a crankshaft including a pair of axially spaced crankpin portions is journaled, a pair of cylinder assemblies guidingly supported from said block structure with said assemblies extending normal to the axis of rotation of said crankshaft and said assemblies mounted for guided lateral shifting transversely of said crankshaft, said cylinder assemblies including central opposite side peripherally extending slots through which axial portions of said crankshaft pass, a pair of double-ended pistons reciprocal in said cylinders and journaled on said crankpin portions, said cylinder assemblies each including opposite end walls through each of which inlet and outlet openings are formed, said block structure including opposing portions between and relative to which the opposite end walls of said cylinder assemblies are slidable, each of said opposing portions including (1) an intake port with which the corresponding inlet openings of one of said assemblies are registrable, (2) an exhaust port with which the corresponding outlet openings of the other of said assemblies are registrable, (3) and a combined compression, combustion and transfer passage including inlet and outlet ends with which the outlet and inlet openings of the corresponding end walls of said one and other cylinder assemblies, respectively, are registrable, each of said opposing portions of said block including volume varying means operative to adjustably vary the effective volume of the corresponding transfer passage.
 2. The combination of claim 1 wherein said opposing portions comprise removable cylinder head portions of said block structure.
 3. The combination of claim 1 wherein said other cylinder assembly includes integral opposite end exhaust outlet passages including inlet ends opening through a side wall portion of said other assembly at their inlet ends, said opposing portions including connecting exhaust ports having inlet ends and outlet ends opening into the first-mentioned exhaust ports, the outlet ends of said exhaust outlet passages being registrable with the inlet ends of said connecting exhaust ports, the inlet ends of said exhaust outlet passages being positioned to be uncovered by the opposite ends of the double-ended piston in said other cylinder assembly, said opposite ends closely approaching their inwardmost limits of movement away from the adjacent opposing portions of said block structure.
 4. The combination of claim 1 wherein the crankpin portion upon which the piston in said one cylinder assembly closely trails the other crankpin portion relative to the direction of intended rotation of said crankshaft.
 5. The combination of claim 1 wherein the inlet ends of said transfer passages include check valve means for preventing reverse flow of compressed gases from said transfer passages into said one cylinder assembly.
 6. The combination of claim 1 wherein each of said transfer passages includes a branch passage intermediate its opposite ends, said branch passages each having a piston slidably disposed therein, said volume varying means comprises shifting means operative to shift said pistons in said branch passages toward and away from the ends thereof opening into said transfer passages.
 7. The combination of claim 6 wherein said shifting means includes fluid supply and vent means operative to selectively admit fluid under pressure into and vent fluid pressure from the portion of each of said branch passages behind the pistons therein. 